1. Field of the Invention
The invention relates to the field of thin film optical mode converters.
2. Prior Art
Mode converters are desirable for use in integrated optics systems in order to couple different types of "devices" which operate with waves having different modes. In addition, non-reciprocal mode converters are desirable in order to form isolators, circulators and so forth.
The Faraday effect may be used to provide mode conversion in thin film optical waveguides. With most materials, once a beam of coherent light has been introduced into the waveguide, its makeup (T.sub.E and T.sub.M modes) will not change as the light wave propagates through the waveguide. However, in a magnetic material which displays the Faraday effect, mode conversion (the conversion of a T.sub.E mode to a T.sub.M mode or vice versa) can occur depending on the alignment of the magnetization of the material with respect to the light propagation direction. When the magnetization of the material is aligned parallel to the light propagation direction, the Faraday effect is at a maximum and will produce a maximum quantity of mode conversion per unit length for that material. When the magnetization of the material is aligned perpendicular to the direction of light propagation, the Faraday effect is at a minimum and no mode conversion is induced. When the magnetization of the material is aligned at an angle between 0.degree. and 90.degree. with respect to the direction of light propagation, an intermediate amount of mode conversion is provided, the amount decreasing as the angle increases from 0.degree. toward 90.degree..
Reciprocal mode converters have been described in which the wave to be converted passes through a material exhibiting the Faraday effect. The magnetization in the Faraday material is made everywhere parallel to the direction of propagation, however, since the T.sub.E and T.sub.M waves have different phase velocities in the Faraday effect material, the direction of the magnetization is periodically reversed in order that mode conversion may accumulate. Such systems are difficult to produce in actual materials because of the difficulties in reversing the direction of the magnetization. Non-reciprocal mode converters have been described in which the T.sub.E and T.sub.M modes must both have the same phase velocity. In order for the T.sub.E and T.sub.M to have the same phase, a degenerate mode is required. The requirement for a degenerate mode can be expected to complicate the fabrication of devices and to limit the materials from which the device may be fabricated. In a first non-reciprocal mode converter, the wave is transmitted within an anisotropic waveguide which is disposed between a magnetic Faraday effect substrate and an anisotropic crystal. In a second non-reciprocal mode converter, the waveguide is magneto-optic and has an anisotropic crystal disposed thereover.